There are surely many more job opportunities out there
for those versed in Windows vs those versed in linux.

That's an incredibly nearsighted basis for recommending Windows,
George. If these were High School seniors I might have a little more
tendency to not balk at this.

I've been doing UNIX & Linux work professionally for almost 6 years now.

I've been doing windows work professionally for over 9 years now.

The things I was doing 9 years ago in Windows are obsolete. The things
I was doing 5 years ago in Windows are now largely obsolete. But you
know what, I'm still doing the same things in Linux & UNIX that I was
doing early on in that part of my career (plus some new things, too).
The things I've learned with Linux have applied amazingly well when
dealing with other vendors systems like Solaris, AIX and HP-UX. I have
no reason to believe that the core of Linux will be all that much
different in another five years. Whereas Microsoft has grandiose plans
for totally revamping how we think of Windows every 4 or 5 years.
Indeed, Windows 2000 is about to become largely obsolete within the
Microsoft community.

Considering the age of these children, the staying power of UNIX, and
the questionable future of Windows (or should I say the complete
assurance that Windows will be completely changed when these kids are
out of high school) I think that putting some flavor of UNIX or Linux
on these machines is a good thing.

Though if the goal were to teach them UNIX, I'd favor NetBSD or OpenBSD
over Linux.